This invention relates to a gas-blowing tuyere useful in production of metal alloys. Particularly, this invention relates to a corrosion-resistant tuyere useful at low gas flow rates and a method of blowing which minimizes corroding of the tuyere and minimizes the gas flow necessary to cool the tuyere tip.
In the production of metal alloys of various compositions, such as silicon steels and stainless steels, it is known to employ tuyeres for purposes of injecting gas into the molten metal, such as for deoxidation, decarburization, desulfurization and stirring. Typically, the tuyeres protrude through a refractory lining of a basic oxygen furnace (BOF), ladle or tundish. Usually, a plurality of tuyeres is used in order to insure the proper amount of gas injection into the molten metal to carry out the desired process of decarburization, desulfurization or other. Furthermore, the tuyeres may be located at any location along the sidewalls or bottom of the vessel, though preferably, the tuyeres in the BOF are located adjacent the bottom portion of the vessel. Generally, the tuyere is constructed of a material which is resistant to attack by molten metal and slag at normal operating temperatures.
At a given flow of inert gas, such as argon, through the tuyere, there is a "critical bath temperature" at which the tip of the tuyere reaches the melting point of the material from which the tuyere is made and begins to melt. Below this critical bath temperature, the tip of the tuyere tubing is cooled sufficiently by the flowing gas so that a small amount of molten metal freezes on the tip of the tuyere. Such a frozen layer of metal (also known as "mushroom") is desirable, for it protects the tuyere from attack by the remaining molten metal in the bath while only slightly affecting the gas flow through the tuyere. Above the critical bath temperature, however, the tuyere melts. The rate of melting is dependent upon several factors, including the temperature of the bath, the gas flow rate and the particular construction of the tuyere.
Attempts at new tuyere designs have been made in order to improve the corrosion resistance of the tuyeres which are subjected to the harsh environment of molten metal baths. One proposed tuyere design comprises an outer metal tube having an inner solid core concentrically spaced within the outer tube and defining a substantially uniform annulus between the core and the outer tube. The inner core consists of a smaller diameter sheath tubing filled with a refractory material. Even such a tuyere has its problems, for it can corrode catastrophically when operated at low gas flow rates, such as less than 150 scfm (4.24 m.sup.3 /min) and particularly at low gas flow rates per unit area of the tuyere of less than 250 scfm/in.sup.2 (0.01 m.sup.3 /min-mm.sup.2) of tuyere annulus area. The corroding and melting of the tuyere becomes particularly acute when high conductivity refractories in the tuyere core and in the lining of the vessel are used. For such reasons, the tuyeres of the prior art have not been used in processes requiring low gas flow rates, and particularly low gas flow rates per unit area of the tuyere annulus, and in designs requiring high conductivity refractories. Furthermore, the prior art does not address tuyere designs which give particular attention to the materials of the tuyere, the construction of the tuyere, the size and gauge of material used in tuyere designs, and the range of minimum to maximum flow rates over which a tuyere is useful.
The abbreviation "scfm" refers to standard cubic feet per minute.
What is needed, therefore, is a tuyere which minimizes excessive corrosion or melting at relatively low gas flow rates, and particularly at low gas flow rates per unit area of the tuyere. Such tuyere designs should also have improved corrosion resistance when high conductivity refractories are used in the tuyere and in the wall lining of a vessel for molten metal. A tuyere and method of blowing gas through the tuyere should have improved cooling of the tuyere tip below its melting point, be useful at low flow rates per unit of area of tuyere and over a wide range of flow rates.